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General Information

The aircraft has four Allison T56-A-14 Turboprop engines rated at 4,600 SHP. They are single-spool, axial-flow, gas turbine engines that operate at a constant speed. Power from the engines is converted to thrust via 4-bladed Hamilton Standard props, with some additional thrust produced from high-velocity exhaust gas. The engine consists of a power unit attached by interconnecting structures to a reduction gear box (RGB). The power unit consists of a compressor section, combustion section, turbine section and accessories section. The RGB has a reduction ratio of 13.54:1 that reduces the 13,820 rpm normal operating speed to 1,020 rpm at the prop shaft. The RGB also has an accessories section
  • 4 turbine stages
  • 14 compressor stages
  • Can-annular
  • Open oil system (oil can move to and from RGB and Engine)
  • 18 thermocouples read the temperature and the average is shown as TIT
  • Operates at constant rpm in flight
    • Design speed is 13,820 RPM, or 100% rpm
  • Power increase initiated by moving power levers, which increases fuel flow, which speeds up engine, which initiates propeller governor control, which increases blade angle and keeps engine at constant speed


  • 14 stage axial compressor 
  • Compression ratio of 9.64:1 
  • Bleed air valves in 5th and 10th stages provided for stabilization during starts and acceleration by unloading back pressure 
    • BAVs are closed by 14th stage bleed air at 94% rpm (controlled by speed sense control) 
  • Approximately 6,000 HP required to turn compressor at 100%rpm 
  • 25% air inlet is used for combustion, 75% for cooling and flame control 
  • Uses of 14th Stage Bleed Air ( BEE COW )
B - Bomb bay heating
E - Engine anti-ice
E - Engine starting
C - Closes 5th and 10th stage bleed air
O - Oil cooler augmentation
W - Wing de-ice


On aft end of compressor and serves the following:
  1. Straightens airflow and distributes it evenly to 6 combustion liners (burner cans) 
  2. Contains ports for bleeding 14th stage compressor air 
  3. Supports the forward end of combustion liners 


  • Can-annular design with 6 combustion liners (cans) 
  • Each has a fuel nozzle at the forward end 
  • Ignighters are in liners 2 and 5 and the flame is carried over to the other cans via crossover tubes 
  • 2 combustion liner drain valves at the bottom 
    • Automatically close at 2-4 psi during start and open at shutdown when pressure back below 2-4 psi 
    • Valves drain off unburned fuel, which exits aircraft via nacelle gang drain (to atmosphere) 


  • Power extracted is transmitted through the engine driveshaft to drive compressor and RGB 
    • All interconnected, not independent 
  • 4 turbine sections (first 3 are shrouded at the ends to limit airflow around blades) 
  • 18 thermocouples (dual element, one positively charged, one negatively, and creates a link for electron flow) measure temperature at turbine inlet (Turbine Inlet Temp = TIT), inputs are sent to TIT gauges and to the Temp Datum system 
    • Thermocouples are wired in series, so if one lost (reads 0) then efficiency goes down, and if one shorts the efficiency goes up 
  • Turbine section cooled by compressor discharge air 

Accessory Drive

  • Located below air inlet housing 
  • Driven by main engine shaft 
  • Accessories mounted on Accessory Section ( SOME FSS )
S - Scavenge oil pump
O - Oil filter
M - Main oil pump
E - Engine driven fuel pumps (centrifugal, primary, secondary)
F - Fuel control unit
S - Speed sensor control
S - Speed sensor valve

*Another memory aid is FESSMOS

Speed Sense Control

  • Located on Accessory section 
  • Contains three switches that actuate at 16, 65 and 94% rpm 
    • Actuated by self-contained flyweight assembly 
  • 16% Switch: 
    • Opens the fuel shutoff valve through fuel shutoff CB (SEDC) if fuel and ignition switch on 
    • Energizes ignition relay through fuel shutoff valve CB (SEDC) 
      • Starter plugs in cans 2 & 5 fire
      • Fuel manifold drain valve solenoid energizes, closing drain 
      • Fuel pumps go in to parallel
    • If prime selected, enrichment valve solenoid energizes 
  • 65% Switch: 
    • De-energizes the ignition relay, which: 
      • De-energizes igniter plugs 
      • De-energizes paralleling valve solenoid that puts primary and secondary pumps in series 
      • De-energizes the fuel manifold drain valve solenoid, but drain held closed via fuel pressure 
  • 94% Switch: 
    • Above 94% rpm, TD control system switches from start limiting to temperature limiting 
    • Also speed sense valve opens and uses 14th stage bleed air to close the 5th and 10th BAVs 

Speed Sense Control Malfunction

  • May be identified by TIT limited to 830°C 
  • If speed sense control malfunction suspected, place affected TD switch to NULL 
  • Do not shut down engine with intent to restart 
  • Pull Fuel Shutoff Valve CB (SEDC) and reset before shutting engine down 

Speed Sense Valve

  • 5th and 10th BAV are controlled by speed sensitive valve 
  • Below 94% rpm, they are held open by compressor air from their respective stage 
  • Above 94% rpm, the speed sensitive valve ports 14th stage bleed air to close them 

Interconnecting Structure

  • Located between power section and reduction gearbox (RGB) 
  • Consists of: 
    • Torquemeter assembly 
    • Safety Coupling 
    • Tie Struts 
  • Purposes: 
    • Transmit power to RGB 
    • Connect RGB and power sections together 

Torquemeter Assembly

  • Consists of inner shaft, outer shaft, housing and shroud (heated by bleed air for anti-ice) 
  • Inner shaft rotates and twists proportional to torque applied 
  • Outer shaft is reference, it rotates but does not twist as it is only connected on one end (to the power section), other end supported on roller bearings 
  • Magnetic torquemeter pickups at the forward end measure the difference between the shafts for gauges in flight station 
    • Pickups are 9° offset from each other
      • WARNING: Very rapid power lever movement should be avoided, as torquemeter “flopover” can occur with too rapid an application of power. This means that at least 5,300 SHP has been momentarily exceeded and the engine(s) shall be inspected as soon as practicable 
    • If Flopover occurs, gauge will indicate false power loss until the power lever is retarded 
    • Power lever must be retarded immediately 

Safety Coupling

  • Transmits power from inner torque shaft to RGB 
  • Designed to limit very high negative torque caused by prop/RGB driving the engine which causes excessive loads on the structure 
  • Negative torque greater than that required to actuate NTS overcomes spring tension (springs within keep it coupled during normal operation) and decouples the RGB from torquemeter shaft 
  • Decouple can occur between -500 and -1700 SHP 
  • Indications of decouple: 
    • Zero or “wandering” SHP 
    • RPM = 100% rpm 
    • TIT = 550°C 
    • FF = 600 lbs per hr 
  • May “recouple” if speed of prop/RGB and power section become equal (never has happened in real life, so not likely) 
  • NATOPS says do not do an in-flight restart after decouple 

Reduction Gear Box (RGB)

  • Converts high speed, low torque energy into low speed, high torque energy for prop 
  • Input from torquemeter assembly is 13,820 RPM, which is converted to 1020 RPM 
    • Reduction ratio of 13.54:1 
    • Accessory Idler Gear is what drives accessories 
  • Items mounted on RGB (“STOGE”):
S - Starter
T - Tach generator
O - Oil pump / filter assembly
G - Generator (engines 2, 3, 4)
E - Engine driven compressor (engines 2, 3)
  • Propeller brake also on RGB 
    • Prevents rotation when engine feathered or when engine is shut down on ground 
    • Also reduces deceleration time following shutdown on ground 

Emergency Shutdown Handle (E-handle)

Functions (“4 F’s and a BOGE”)

F - Feather system actuates (E)
F - Feather valve moves to feather position (M)
F - Fuel at fuel control shuts off (EM)
F - Fuel tank emergency shutoff valve closes (M)
B - Bleed air shutoff valve closes (E)
O - Oil tank shutoff valve closes (E)
G - Generator cooling air shutoff valve closes (M)
E - EDC dumps and firewall shutoff valve closes (E)

* E: Electric actuation    M: Mechanical actuation

Note: The electrical functions of the emergency shutdown handle are controlled by electrical switches powered by the MEDC bus. The system is redundant, providing two switches and two CBs. (“Wiper switches”)

Engine Failure Emergency Procedure

Execute Emergency Shutdown for any of the following (“VP FOOT C”)
  • Vibration (extreme or abnormal) 
    • WARNING: In rare cases, significant airframe vibration may be indicative of impending prop blade failure. This vibration may be accompanied by noticeable/profound vibration of the power lever and/or E-handle 
  • Power loss (excessive or uncontrollable) 
  • Fire 
  • Oil temperature rise (sudden or uncontrollable) 
  • Oil pressure low or excessive 
  • TIT increases and cannot be controlled 
  • Chips light with secondary indications
Note: The FE should lock out the fire warning horn prior to executing the Emergency Shutdown procedure

Note: Avoid operating the aircraft at high power settings, high AOA and low airspeeds, as this conditions may induce a fire warning (1.52 Vs clean will keep plane at or below 12 AOA)

Note: To minimize buffet of a feathered prop, use the starter to position the blades so they don’t parallel the leading edge

Warning: If the engine has been secured because engine fire or fuel leak, careful consideration should be given before pushing the E-handle back in. Temperatures may be sufficiently high to cause reignition 

Failure of Two Engines in Flight Emergency Procedure

  • Shut down via Emergency Shutdown procedure 
  • If both inboard engines failed, then descend to safe altitude 
  • Propeller synchrophasing not available
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